1. Field of the Invention
The present invention relates to a heat processing apparatus for a semiconductor process for forming a film on a target substrate such as a semiconductor wafer. The semiconductor process refers to various types of processes which are performed to form a semiconductor layer, an insulating layer, a conductive layer, or the like in a predetermined pattern on a target substrate such as a semiconductor wafer or a glass substrate for an FPD (Flat Panel Display), such as an LCD (Liquid Crystal Display), thereby manufacturing a structure including a semiconductor device, a wiring line connected to the semiconductor device, an electrode, or the like on the target substrate.
2. Description of the Related Art
Examples of a semiconductor manufacturing apparatus include a batch type heat processing apparatus such as a vertical furnace or a horizontal furnace. A vertical heat processing apparatus (vertical furnace) is an apparatus in which a wafer boat which holds a large number of target substrates is loaded in a vertical reaction tube and heated. An example of the heat process performed by a heat processing apparatus of this type includes a film formation process by a CVD process at a low pressure.
When performing low-pressure CVD, a method of supplying a film formation gas from the lower portion of a reaction tube and exhausting the gas from its upper portion is conventionally employed. With this method, however, the gas concentration in the upper portion of the reaction tube decreases, and variations in film thicknesses of target substrates in the vertical direction increase. To improve the uniformity of the film thickness (the decomposition amount of the film formation gas in the reaction tube in the vertical direction) among the target substrates, the process field in the reaction tube is vertically divided into zones. Heaters are also divided to correspond to the respective zones. Then, zone control can be performed independently among the zones such that a temperature gradient is formed in the reaction tube.
Some of recent processes may require to eliminate or minimize the temperature gradient among the target substrates. An example of such a process includes one which forms a silicon nitride film (SiN film) to be disposed on a certain portion of a semiconductor device.
According to a CVD process that forms an SiN film, for example, SiH2Cl2 (dichlorosilane) gas and NH3 (ammonia) gas are supplied into the reaction tube. In this case, in order to decrease variations in concentration distribution of the process gases in the reaction tube, there is known a technique in which a large number of holes are formed in a gas nozzle in the reaction tube and a process gas is supplied through the respective holes, as described in, e.g., Patent Document 1 (see Jpn. Pat. Appln. KOKAI Publication No. 2004-260204 ((0014), FIG. 2)). In this case, however, in order to decrease the pressure (flow rate) gradient in the vertical direction of the process gas supplied from the holes of the gas nozzle, the gas pressure in the gas nozzle must be increased. Also, in this case, as the opening diameters of the holes become as very small as, e.g., 1 mm or less, a product generated by the reaction of the process gas tends to deposit on the opening edges of the holes during the heat process of the target substrates. This may change the hole diameters. If the uniformity in gas flow rate among the holes is lost, it becomes difficult to form films with high uniformity among the target substrates. Furthermore, the product deposited in the holes may be dropped by the pressure of the process gas and cause particle contamination. Therefore, cleaning must be performed frequently.
For example, when the opening diameters of the holes of the gas nozzle are increased, the gas flow velocity may become low and variations in gas flow velocity may decrease. In order to achieve this, the pipe diameter of the gas nozzle must be increased, and the pressure gradient in the gas nozzle in the vertical direction must be decreased (the pressure must be decreased). Consequently, the gap between the reaction tube and the target substrates increases, and the planar uniformity of the film thickness degrades.
Jpn. Pat. Appln. KOKAI Publication No. 2003-203871 ((0010), FIG. 1) describes a technique to solve this problem, in which the wall surface of a reaction tube is projected outwardly only at a part where a gas nozzle is to be installed, so that the gap between the inner wall of the reaction tube and target substrates decreases. However, as the distance between the wall surface of the reaction tube and the outer surfaces of the target substrates is not constant, the planar uniformity of the film thickness degrades. Also, because the pressure resistance of the reaction tube decreases, the reaction tube must have a double structure. In this case, the cleaning time in the reaction tube prolongs, and the apparatus becomes bulky.
A technique is also known in which gas supply pipes with different heights are arranged in the reaction tube so as to decrease variations in concentration of the process gas. However, this technique requires, e.g., five or more gas supply pipes, undesirably leading to a large-scale process gas supply facility.
Jpn. Pat. Appln. KOKAI Publication No. 2000-299287 ((0023), FIG. 15) describes a heat processing apparatus in which a path forming member is provided outside a reaction tube and a gas is supplied into the reaction tube through the path forming member. When the gas flow rate is small (an amount used for an ordinary heat process), however, a flow rate gradient in gas is formed in each slit. As the gap between the reaction tube and target substrates is not constant, the planar uniformity of the film thickness degrades.
Jpn. Pat. Appln. KOKAI Publication No. 8-186081 ((0044), FIG. 11) describes a heat processing apparatus in which an exhaust port is formed at a portion opposing a gas supply region, and a horizontal gas flow is formed. As the side wall of the reaction tube projects outwardly to form the exhaust port, the planar uniformity of the film thickness also degrades.